In reality, a measure of the total opposition to current flow in an alternating current circuit, made up of two components, ohmic resistance and reactance, and usually represented in complex notation as Z = R + iX, where R is the ohmic resistance and X is the reactance. In this car audio, typically we use impedance as only the resistance, to make speaker selection easier. This is how many "Ohms" a speaker "has".

The amount of audio power produced by a speaker with one watt of input. At Bazooka, we measure this in the real world application--in the car, with the microphone 26 inches above the seat, approximating the Drivers head position.